A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
Lithography is widely recognized as one of the key steps in the manufacture of ICs and other devices and/or structures. However, as the dimensions of features made using lithography become smaller, lithography is becoming a more critical factor for enabling miniature IC or other devices and/or structures to be manufactured.
A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in equation (1):
                    CD        =                              k            1                    *                      λ                          N              ⁢                                                          ⁢              A                                                          (        1        )            where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection system used to print the pattern, k1 is a process dependent adjustment factor, also called the Rayleigh constant, and CD is the feature size (or critical dimension) of the printed feature. It follows from equation (1) that reduction of the minimum printable size of features can be obtained in three ways: by shortening the exposure wavelength λ, by increasing the numerical aperture NA or by decreasing the value of k1.
In order to shorten the exposure wavelength and, thus, reduce the minimum printable size, it has been proposed to use an extreme ultraviolet (EUV) radiation source. EUV radiation is electromagnetic radiation having a wavelength within the range of 5-20 nm, for example within the range of 13-14 μm. It has further been proposed that EUV radiation with a wavelength of less than 10 nm could be used, for example within the range of 5-10 nm such as 6.7 nm or 6.8 nm. Possible sources include laser-produced plasma (LPP) sources, although other types of source are possible.
An example of current progress in the development of LPP sources for EUV lithography is described in the paper “High power LPP EUV source system development status” by Benjamin Szu-Min Lin, David Brandt, Nigel Farrar, SPIE Proceedings Vol. 7520, Lithography Asia 2009, December 2009 (SPIE Digital Library reference DOI: 10.1117/12.839488). In a lithographic apparatus, the source apparatus will typically be contained within its own vacuum housing, while a small exit aperture is provided to couple the EUV radiation beam into an optical system where the radiation is to be used.
In order to be useful in high-resolution patterning for lithography, the EUV radiation beam must be conditioned to obtain desired parameters such as uniformity of intensity and angular distribution, when it reaches the reticle. Examples of an illumination system are described in United States Patent Application Publication Nos. US 2005/0274897A1 (Carl Zeiss/ASML) and US 2011/0063598A1 (Carl Zeiss). The example systems include a ‘fly's eye’ illuminator which transforms the highly non-uniform intensity profile of the EUV source into a more uniform and controllable source.
For good imaging performance it should also be ensured that the conditioned EUV radiation beam is uniform in intensity, particularly in a non-scanning direction (as explained further below). The known illumination systems include various techniques for uniformity correction, removing residual non-uniformities that are not canceled by the fly's eye illuminator. The known techniques may not be able to correct for all variations in the EUV radiation beam. For example, fluctuations in the position of a fuel droplet relative to the timing and place of a laser pulse may cause asymmetry in the radiation beam that are not canceled by the fly's eye illuminator, but are too rapid to be corrected conveniently by the other uniformity correction mechanisms known to date.